The performance characteristics desired in retortable packaging materials and in packaging bags that must be sterilized under pressurized treatment, such as intravenous bags (IV bags), include transparency to allow the contents to be checked, flexibility to enable liquid discharge without forming an air vent, low-temperature impact resistance so that the bag does not rupture during low-temperature storage and low-temperature transport to preserve the quality of the contents, heat resistance so that deformation and fusion do not occur even when sterilization at 121° C. is carried out, and fabricability such as heat-sealability to facilitate bag-making.
With regard to IV bags in particular, vinyl chloride resins were formerly used as a material that satisfies the above performance characteristics. However, owing to the leaching out of plasticizers and waste disposal problems, and also to recent concerns over the global environment, such resins have been replaced with polyolefin resins.
IV bags composed primarily of polyethylene, though endowed with an excellent flexibility and impact resistance, have a poor heat resistance and thus give rise to appearance defects such as deformation at a sterilization temperature of 121° C. (overkill conditions), making them incapable of functioning satisfactorily as IV bags (see, for example, Patent Document 1). On the other hand, IV bags composed primarily of polypropylene have a good heat resistance, but are hard as an IV bag material and have an inadequate impact resistance at low temperatures, as a result of which these too are unable to satisfy the above performance characteristics (see, for example, Patent Document 2).
Art has thus been disclosed in which flexibility and impact resistance are conferred by the addition of an elastomeric component to polypropylene (see, for example, Patent Document 3). However, problems with this approach are that the heat resistance of polypropylene is sacrificed, low-molecular-weight ingredients bleed out following sterilization, and the transparency worsens. Art involving the addition of a styrene-based elastomer as the elastomeric component has also been disclosed (see, for example, Patent Document 4), but blocking tends to arise and the productivity leaves much to be desired. Moreover, styrene-based elastomers are more expensive than olefinic elastomers, leading to cost-related issues as well.
Unrelated to the above, polypropylene block copolymers in which an elastomeric component is added by continuous polymerization using a Ziegler-Natta catalyst have been developed (see, for example, Patent Document 5). Unsurprisingly, however, bleedout arises following sterilization, and the transparency is poor. Water-cooled blown films composed of a propylene-ethylene block copolymer having an elastomeric component added thereto and obtained by continuous polymerization using a metallocene catalyst have also been disclosed (see, for example Patent Document 6). However, these do not yet have a sufficient low-temperature impact resistance. In addition, films for medical use which contain a heterogeneous blend of resins have been disclosed (see, for example, Patent Document 7), but these too have lacked an adequate impact resistance at low temperature.
Hence, although there exists a need for IV bag materials which strike a good balance among the properties of heat resistance, transparency, flexibility and impact resistance, and which moreover are low-cost, materials satisfying such a need have not previously been found.
Moreover, the IV bag-making process includes the steps of welding injection-molded parts such as a spout, a discharge port and an injection port to the bag, which requires melting of the film for sufficient fusion to take place. For this purpose, heat sealing is carried out under very harsh conditions (e.g., high temperature, high pressure, long duration). In a fully melted state, the molten resin ends up sticking to the sealing bar, inevitably worsening productivity. To address this problem, technology has been disclosed wherein the outer layer and the inner layer of a laminated film are provided with different melting points, enabling the inner layer to be melted while the outer layer remains solid (see, for example, Patent Document 7). The inner layer is made of a polyethylene resin and can thus withstand a sterilization temperature of 115° C.; however, at 121° C. sterilization, the inner faces of the film end up sticking to each other (“internal fusion”). Hence, the heat resistance is inadequate.